The cooling capacity of an air conditioning system including a fixed displacement refrigerant compressor is typically regulated by cycling the compressor on and off. In the exemplary automotive air conditioning system 10 of FIG. 1, the compressor 12 is coupled to a driven pulley 14 by an electrically activated clutch 16 so that compressor 12 can be cycled on and off by respectively engaging and disengaging clutch 16. The refrigerant flows through a closed circuit including a condenser 18, an orifice tube 20, an evaporator 22, and an accumulator/dehydrator 24 arranged in order between the compressor discharge and suction ports 26 and 28. The cooling fans 30 are electrically activated to provide supplemental airflow for removing heat from high-pressure refrigerant in condenser 18, and the orifice tube 20 allows the cooled high-pressure refrigerant in line 30 to expand in isenthalpic fashion before passing through the evaporator 22. The evaporator 22 is formed as an array of finned refrigerant-conducting tubes, and an air intake duct 32 disposed upstream of evaporator 22 houses a motor driven ventilation blower 34 for forcing air past the evaporator tubes. The duct 32 is divided upstream of the blower 34, and an inlet air control door 36 is adjustable as shown to apportion the inlet air between outside air and cabin air. An air outlet duct 38 downstream of evaporator 22 houses a heater core 40 formed as an array of finned tubes through which flows engine coolant. The heater core 40 effectively bifurcates the outlet duct 38, and a re-heat air control door 42 next to heater core 40 is adjustable as shown to apportion the airflow through and around heater core 40. The heated and un-heated air portions are mixed in a plenum 44 downstream of heater core 40, and two discharge air control doors 46 and 48 are adjustable as shown to direct the mixed air through one or more outlets, including a defrost outlet 50, a heater outlet 52, and driver and passenger panel outlets 54 and 56. Activation of compressor clutch 16, cooling fans 30, blower 34, and air control doors 36, 42, 46 and 48 is controlled by a microprocessor-based controller 58.
Traditionally, the controller 58 is programmed to cycle the compressor on and off as required to prevent condensate from freezing on the evaporator 22, and a portion of the conditioned air is re-heated by heater core 40 so that the temperature of air discharged through the outlets 50-56 corresponds to a desired discharge air temperature. The compressor cycle control can be achieved with a pressure transducer responsive to the low side refrigerant pressure, or with a temperature transducer 60 responsive to the evaporator outlet air temperature (Tevp). In either case, the compressor clutch 16 is disengaged when the measured parameter falls below a calibrated lower threshold, and is later re-engaged when the measured parameter rises above a calibrated upper threshold. For example, the upper and lower thresholds may be calibrated so that Tevp cycles between 3° C. and 4.5° C., establishing a hysteresis band of 1.5° C.
More recently, it has been proposed to improve the system efficiency by varying the compressor capacity control based on user cooling requirements. In this way, the compressor capacity can be reduced to satisfy the occupant cooling requirements with a somewhat elevated evaporator outlet air temperature (or refrigerant pressure), thereby reducing both over-dehumidification of the discharge air and series re-heating of the evaporator outlet air. See, for example, the U.S. Pat. No. 6,293,116 to Forrest et al., assigned to the assignee of the present invention, and incorporated by reference herein. The general principle is to cool the inlet air only as low as needed to meet the discharge air temperature requirement. For example, if the discharge air temperature target is 10° C., there is no need to cool the air down to 3° C., only to reheat it to 10° C. To provide at least a certain level of dehumidification for occupant comfort and prevention of windshield fogging, the evaporator temperature set point can be kept below a limit value such as 10° C. But in general, reducing over-dehumidification improves occupant comfort, and operating the compressor at a reduced capacity improves the energy efficiency of the air conditioning system. This control can be achieved with an electronically controlled variable displacement compressor, but it is generally more cost effective to use a fixed displacement compressor that is cycled on and off to control cooling capacity. Another possibility is to cycle a pneumatically controlled variable displacement compressor, as disclosed by Zima et al. in the U.S. patent application Ser. No. 11/805,469, filed May 22, 2007, assigned to the assignee of the present invention, and incorporated by reference herein.
In systems where the compressor capacity is controlled by cycling, the calibrator establishes a hysteresis band defined by upper and lower switching thresholds as mentioned above. In the case of the traditional freeze-point control, the set point (i.e., the lower threshold) is fixed at 3° C., for example, whereas in the case of the high-efficiency control, the set point varies between, say, 3° C. and 10° C. In either case, the difference between the upper and lower thresholds (i.e., the hysteresis band) is selected to strike a balance between the compressor clutch cycling frequency (which increases as the difference in thresholds is reduced) and discharge air temperature variation (which increases as the difference in thresholds is enlarged). In general, the calibrator seeks to limit the compressor clutch cycling frequency to address compressor and clutch durability considerations, while limiting the discharge air temperature variation to address occupant comfort considerations. This is graphically illustrated in FIGS. 2A-2B. FIG. 2A illustrates a freeze point control in which the compressor is cycled on and off using a fixed temperature set point 60 of 3° C. following an initial cool-down period. The set point of 3° C. serves as a lower threshold, and the upper threshold 62 is calibrated to 4.5° C. for a hysteresis band of 1.5° C. FIG. 2B illustrates a high efficiency control in which the compressor is cycled on and off about a variable temperature set point 64 following the initial cool-down period. In the illustration, the set point 64 has an initial value of 3° C., and then transitions to an elevated value of about 8.0° C. Similar to FIG. 2A, the set point 64 serves as a lower threshold, and an upper threshold 66 tracks the set point 64 to define a hysteresis band of 1.5° C. Thus, the width or size of the hysteresis band can be the same for both control strategies.
A problem faced in the calibration of compressor switching limits (i.e., the hysteresis band) is that the settings which provide an adequate tradeoff between compressor cycling frequency and discharge air temperature variation under one set of operating conditions can fail to provide an adequate tradeoff under a different set of operating conditions. Accordingly, what is needed is a way of achieving an optimal or specified tradeoff between compressor cycling frequency and discharge air temperature variation under any set of operating conditions.